The variability of surface pCO_1tn2 and nutrients in the North Atlantic Ocean [Elektronische Ressource] / von Heike Lüger

The Variability of Surface pCO and Nutrients in the 2North Atlantic Ocean Dissertation zur Erlangung des Doktorgrades der Mathematischen-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel von Heike Lüger Kiel 2003 Referentin: Prof. Dr. Karin Lochte Korreferent: Prof. Dr. Douglas Wallace Tag der mündlichen Prüfung: 05.02.2004 Zum Druck genehmigt: Kiel, den Der Dekan […] since every piece of matter in the Universe is in some way affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation – every sun, every planet, their orbits, their composition and their economic and social history from, say, one smal piece of fairy cake. Douglas Adams: “The Restaurant at the End of the Universe” Für meine Mutter und für meine Oma! Abstract This PhD thesis was part of the EU-funded project CAVASSOO (Carbon Variability Studies by Ships of Opportunity). The major goal of the project was to establish an international network in the North Atlantic consisting of commercial vessels that are equipped with pCO 2measurement devices. The installation of a autonomously working pCO unit onboard the 2carcarrier M/V Falstaff was completed in January, 2002.

[…] since every piece of matter in the Universe is in some way affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation – every sun, every planet, their orbits, their composition and their economic and social history from, say, one smal piece of fairy cake.

Douglas Adams: “The Restaurant at the End of the Universe”

Für meine Mutter und für meine Oma!

Abstract

This PhD thesis was part of the EU-funded project CAVASSOO (Carbon Variability Studies by Ships of Opportunity). The major goal of the project was to establish an international network in the North Atlantic consisting of commercial vessels that are equipped with pCO 2measurement devices. The installation of a autonomously working pCO unit onboard the 2carcarrier M/V Falstaff was completed in January, 2002. Measurements started a month later with the first transatlantic crossing. In the following months continuous and discrete samples were analyzed. It was examined whether it is possible to correlate the oceanic pCO with 2parameters that can be retrieved by ship-independent observations such as remote sensing. The correlatin between pCO and nitrate was promising in this context and it could also be 2shown that nitrate correlated well with the mixed layer depth. Parameters such as temperature and chlorophyll on the other hand did not reveal a unique correlation with the pCO . Within 2this thesis it was also shown that the seawater pCO in the eastern basin (10°W-35°W) 2showed smaller seasonal changes than in the western basin (36°W-70°W) in the North Atlantic. This was explained by the fact that in the eastern basin the temperature effect on the seawater pCO was counteracted by the biological effect yielding a damped seasonal pCO 2 2cycle. In the western basin, however, temperature was the major force on the pCO which was 2not reduced by a counteracting biology effect thus yielding a pronounced seasonal pCO 2cycle. The CO flux calculation showed that this region of the North Atlantic was a sink for 2atmospheric CO in 2002. When comparing the CO flux to a well-cited pCO climatology the 2 2 2difference was small (4%). The seasonal cycles of nutrients within different watermasses showed distinct patterns. The C:N ratio of the seasonal new production were similar to the Redfield ratio for all watermasses excecpt for the Gulfstream watermass. In the latter a carbon overconsumption with respect to Redfield could be shown which pointed at N fixation. This 2result was underlined by the high N:P values of the Gulfstream waters.

The global climate change is inseparately connected to the carbon cycle with carbon dioxide as one of the major greenhouse gases. It is a well known fact that anthropogenic emissions have led to a higher atmospheric content of CO since the onset of the industrial revolution. 2Such a high atmospheric CO concentration is unprecedented and has not been exceeded 2during the past 420,000 years (Houghton et al., 2001). The atmosphere, however, only stores about half of the anthropogenic CO and it remains unclear what happens to the other half. 2Generally, the atmosphere exchanges CO in a source/ sink pattern with two major reservoirs: 2the terrestrial biosphere and the ocean. The atmospheric CO content is well known for two 2reasons. Firstly, in the atmosphere CO is distributed uniformly due to the rapid mixing and, 2secondly, a high quality world-wide network exists which continously monitors the atmospheric CO content (Conway et al., 1994). On land and in the ocean the CO variability 2 2and consequently carbon storage is much more difficult to determine. A vast multitude of carbon species exists in the terrestrial biosphere and land use is continously changing which makes it very difficult to constrain carbon storage (Wallace, 2001). In the ocean most of the carbon is present as inorganic carbon and the carbon uptake term can be estimated with higher reliability than in the terrestrial biosphere. Thus the latter is commonly calculated from the difference between atmospheric and oceanic reservoir. A lot of research has been conducted to find out about the general mechanisms that underlie the complex carbon cycle in the ocean. The North Atlantic Ocean plays an important role in the global ocean with regard to the uptake of anthropogenic CO . Deep-water formation in the high northern latitudes of the 2Atlantic leads to a deep penetration of anthropogenic CO (Gruber, 1998). 2Based on this scientific knowledge the European project CAVASSOO (Carbon Variability Studies by Ships of Opportunity) was launched as a pilot project in 2000. Within my thesis - which was part of this project - I came across major questions and topics that are crucial for a better understanding of the carbon cycling and global implications such as climate change: • How does the marine carbon cycle work? What are the patterns of CO within the 2ocean and at the surface? • How variable is the CO flux and related parameters in the North Atlantic Ocean? 2• How well is the international database established? How important is the addition of new CO data and parameters such as e.g. wintertime nutrients? 2

areas where the partial pressure of CO is higher in the ocean than in the atmosphere the flux 2will be from the ocean into the atmosphere – this pattern is called outgassing, i.e. the ocean acts as a source of CO for the atmosphere. Hence, the reverse process denotes the ocean to 2act as a sink for CO to the atmosphere. The air-sea exchange of CO is difficult to measure 2 2directly and is controlled by myriad processes, e.g.: wind speed, sea state, surface processes, bubble entrainment, bioproductivity (McGillis et al., 2001). The CO flux is mostly derived 2from estimated transfer velocities, solubility of CO , and the difference between the partial 2pressure of CO in the bulk seawater and at the ocean surface (DpCO ). This procedure is a 2 2